An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave. They are widely used in innumerable electronic devices. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by electronic beepers and video games. The harmonic, or linear, oscillator produces a sinusoidal output.
The basic form of a harmonic oscillator is an electronic amplifier connected in a positive feedback loop with its output fed back into its input through a filter. When the power supply to the amplifier is first switched on, the amplifier's output consists only of noise. The noise travels around the loop and is filtered and re-amplified until it increasingly resembles a sine wave at a single frequency.
An oscillator circuit which uses an RC network, a combination of resistors and capacitors, for its frequency selective part is called an RC oscillator. Two configurations are common. One is called a Wien bridge oscillator. In this circuit, two RC circuits are used, one with the RC components in series and one with the RC components in parallel. The Wien Bridge is often used in audio signal generators because it can be easily tuned using a two-section variable capacitor or a two section variable potentiometer (which is more easily obtained than a variable capacitor suitable for generation at low frequencies).
The second common design is called a “Twin-T” oscillator as it uses two “T” RC circuits operated in parallel. One circuit is an R-C-R “T” which acts as a low-pass filter. The second circuit is a C-R-C “T” which operates as a high-pass filter. Together, these circuits form a bridge which is tuned at the desired frequency of oscillation. The signal in the C-R-C branch of the Twin-T filter is advanced, and in the R-C-R branch, delayed, so they may cancel one another for frequency f=1/(2πRC) if x=2; if it is connected as a negative feedback to an amplifier, and x>2, the amplifier becomes an oscillator.
In a crystal oscillator, a piezoelectric crystal (commonly quartz) may take the place of the filter to stabilize the frequency of oscillation. These kinds of oscillators contain quartz crystals that mechanically vibrate as resonators, and their vibration determines the oscillation frequency. Crystals have very high Q-factor and also better temperature stability than tuned circuits, so crystal oscillators have much better frequency stability than RC oscillators. Crystal oscillators are commonly used to stabilize the frequency of radio transmitters, and to generate the clock signal in computers. The Pierce oscillator circuit is often used for crystal oscillators.
In any oscillator circuit, current consumption increases proportional to the system clock frequency. Therefore, keeping the system clock as low as possible is critical to reducing power consumption. The clock frequency is affected by a number of factors and there are heretofore unaddressed needs with previous low power solutions.